The present invention relates to a glass bulb for a cathode ray tube and a cathode ray tube, wherein a face portion has a substantially flat outer surface, and the face portion has a drastically changed wall thickness distribution.
In cathode ray tubes, the envelope comprises a glass bulb, which includes a panel portion for displaying an image, and a funnel portion formed in a funnel shape and having one end provided with a neck portion for housing an electron gun. The panel portion has a substantially box-shaped form and comprises a substantially rectangular face portion for providing a screen (screen area) and a skirt portion extending substantially vertically from a peripheral edge of the face portion to define a side wall.
In recent years, as the panel portion has been employed one wherein the face portion has a substantially flat outer surface and a concave inner surface, and is thermally strengthened to improve strength. In general, in the case of a panel portion for a cathode ray tube with a shadow mask employed therein, in order that a shadow mask can easily control electron beams from an electron gun to project the electron beams on a fluorescent screen on an inner surface of a face portion, or in order that the cathode ray tube can increase strength for the purpose of improving anti-implosion properties, the panel portion has an extremely greater wall thickness in a peripheral portion of the face portion than a central portion thereof. In other words, the panel portion has a wall thickness distribution in the face portion, wherein the wall thickness increases from the central portion toward the peripheral portion. For example, JP-A-11-67124 discloses that the wall thickness of the peripheral portion is not less than 1.5 times that of the central portion.
However, when the panel portion is joined to the funnel portion through an end surface of the skirt portion by frit glass, and the panel portion and the funnel portion thus joined are subjected to a heat treatment process (an evacuation process) to produce a cathode ray tube, the generation of a great thermal stress due to the wall thickness distribution in the face portion has caused fracture, lowering productivity.
When the panel portion has a drastically changed wall thickness distribution in the face portion as stated earlier, the difference in the wall thickness between the central portion and the peripheral portion of the face portion produces a great thermal stress throughout an area from an edge of the face portion to a blend portion and the skirt portion in the heat treatment process. When the panel is equal or is little different in the wall thickness distribution between the central portion and the peripheral portion of the face portion, no thermal stress raises a serious problem. However, in the case of a cathode ray tube wherein the useful screen area has a diagonal diameter of 60 cm, and the wall thickness of the peripheral portion in the face portion is beyond 1.7 times that of the central portion for instance, the thermal stress generated in the panel portion reaches 30-40 MPa. Glass exponentially increases the probability of occurrence of breakage in the range covering these stress values. This means that a slight increase in the thermal stress introduces a significant increase in the occurrence of fracture, greatly adversely affecting productivity.
Although it is the most effective to minimize the nonuniformity in the wall thickness distribution of the face portion in order to reduce the thermal stress, it is impossible to modify the shape of the inner and outer surfaces of the face portion in simple fashion since the shape is an important factor related to image quality or visibility.
In order to restrain the fracture in a glass bulb, there is a method for strengthening the panel portion by thermal strengthening. This method aims at restraining the crack by causing a thermally strengthened compressive stress to remain to cancel the thermal stress. In the case of the panel having a drastically changed wall thickness distribution in the face portion, it is difficult to introduce a strengthening stress in equal fashion since a difference in cooling between the face portion and the skirt portion is apt to be created when glass is cooled and solidified in a glass molding process. From this reason, it is impossible to obtain a sufficient effect to restrain the fracture of the glass bulb. Additionally, unexpected fracture is caused by the unnecessary tensile stress stated earlier in some cases.
Careful consideration has not been given to restraint on the thermal stress in designing the panel portion and the funnel portion. For example, when a thermally strengthened panel portion is combined with a funnel portion to produce a cathode ray tube, it is impossible to restrain the thermal stress in sufficient manner. Optimization has not been provided in terms of reduction in the weight of the glass bulb as well.
When a funnel portion is too thin, the imbalance in heat capacity between the funnel portion and a panel portion is exaggerated. In the heat treatment process, the funnel portion is heated or cooled too rapidly to generate an excess thermal stress, lowering the strength of the funnel portion. Since the funnel portion is provided with a great wall thickness so as to compensate the lowered strength in normal designing, no sufficient reduction in the weight of the funnel portion is provided as a matter of fact.
In consideration of the problems stated earlier, it is an object of the present invention to provide a lightweight glass bulb and a cathode ray tube with the glass bulb employed therein, wherein the thermal stress that is generated in a panel portion with a face portion having a drastically changed wall thickness distribution can be restrained as small as possible, and thermal strengthening is effectively applied.
The present invention has been proposed in consideration of the problems and has been attained by finding that investigation of the thermal stress, which is generated in a panel portion with a face portion having a drastically changed wall thickness distribution in a heat treatment process, reveals that when the wall thickness of a skirt portion and the wall thickness of a portion around the face portion are harmonized with each other, a restraining effect on the thermal stress can be provided, and that the thermal stress can be effectively cancelled to prevent fracture by applying a desired strengthened compressive stress to an outer surface region of a portion extending from an edge of the face portion on a short axis or a long axis to the skirt portion.
Additionally, the present invention has accomplished a lightweight glass bulb by finding that investigation of the relationship of the wall thickness of a funnel portion to the thermal stress generated in a panel portion and the stress generated by evacuating the inner side of a bulb (hereinbelow, referred to as xe2x80x9cthe vacuum stressxe2x80x9d) in a heat treatment process reveals that the wall thickness of the funnel portion is closely related to the thermal stress and the vacuum stress, and that when the wall thickness of the funnel portion is determined in a certain range with respect to the wall thickness of the face portion, the thermal stress can be restrained to prevent fracture while making the funnel portion thinner.
The present invention provides a glass bulb for a cathode ray tube, comprising a panel portion having a rectangular face portion and a skirt portion, the rectangular face portion having a substantially flat outer surface and a concave inner surface, the skirt portion extending substantially vertically from a peripheral edge of the face portion; a funnel portion having one end connected to the panel portion; and a neck portion having connected to the other end of the funnel portion; wherein at least the panel portion being thermally strengthened to apply a compressive stress thereto; the panel portion and the funnel portion satisfy the following conditions 1, 2, 3 and 4;
wherein the panel portion satisfies the following conditions at an edge portion of a useful screen area in the face portion on a short axis or an edge portion of a useful screen area in the face portion on a long axis, whichever has a greater wall thickness:
1) 1.70xe2x89xa6TL/TC, wherein TC is a central wall thickness of the face portion, and TL is a wall thickness of the useful screen area,
2) 5 MPaxe2x89xa6|"sgr"C|xe2x89xa614 MPa, wherein "sgr"C is the value of a strengthened compressive stress in at least an area of a side surface of the skirt portion in the vicinity of a mold match, and
3) 0.43xe2x89xa6TS/TLxe2x89xa60.50, wherein TS is a wall thickness of a sealing end surface of the skirt portion;
the funnel portion satisfies the following condition:
4) 0.37xe2x89xa6TF/TCxe2x89xa60.49, wherein TF is a wall thickness at a portion located at B/2 when B is the length of a body portion in the funnel portion in a bulb axis direction, the body portion extending from a sealing end to a yoke portion thereof.
Additionally, the present invention provides a cathode ray tube produced by employing the panel portion and the funnel portion.
In accordance with the present invention, the difference in cooling between the face portion and the skirt portion can be reduced, and imbalance in the strengthened compressive stress can be controlled to obtain a thermal strengthening effecting in sufficient fashion by specifying the shape of the skirt portion of the panel portion with the face portion having a drastically changed wall thickness distribution as stated earlier. In other words, the present invention is characterized in that the thermal stress that is generated due to the wall thickness distribution of the face portion can be restrained to prevent or reduce the occurrence of fracture in the panel portion by specifying the wall thickness of the skirt portion and the value of a compressive stress given by thermal strengthening.
In accordance with the present invention, the wall thickness of the skirt portion and the value of a compressive stress given by thermal strengthening are expediently specified on the short axis or the long axis of the face portion. This is because fracture is apt to be generated from such a portion. The reason is that although the wall thickness distribution of the face portion has the greatest wall thickness to the edge portion of the useful screen area, the edge portion is affected by the wall thickness distribution of the skirt portion to be subjected a great thermal stress especially on the short axis or the long axis and to make the tensile vacuum stress maximumized in a central portion of the portion around the face portion in a cathode ray tube production process. When the edge portion of the useful screen area on the short axis and the edge portion of the useful screen area on the long axis are compared to each other, the thermal stress is higher in the edge portion having a greater wall thickness.
From this viewpoint, it is important to specify the wall thickness of the skirt portion and the value of a thermally strengthened compressive stress on the edge portion of the useful screen area on the short axis or the edge portion of the useful screen area on the long axis, whichever has a greater wall thickness. Which of the edge portions has a greater wall thickness is variable and varies depending on the type or the shape of the panel portion since the wall thickness of the useful screen area on the short axis or the long axis is determined by an aspect ratio of the useful screen area or a design value given to the inner shape of the face portion. When a required wall thickness and the value of a thermally strengthened compressive stress have been specified with respect to the edge portion on the selected short axis or long axis, a required wall thickness and the value of a thermally strengthened compressive stress given to the other portions in the panel portion may be determined in the same way or be designed based on the conditions thus specified. This is also applicable to the funnel portion.